1. Field of the Invention
The present invention relates to an aberration correction method and an aberration correction apparatus.
2. Description of the Related Art
In recent years, as an image photographing apparatus for ophthalmic application, a scanning laser ophthalmoscope (SLO) and an imaging apparatus employing interference of low coherent light have been developed. The SLO two-dimensionally irradiates a fundus with laser light and receives reflected light reflected on the fundus to create an image of the fundus. The imaging apparatus employing the interference of the low coherent light is called an optical coherence tomography (OCT), which is used, in particular, to obtain a tomographic image of the fundus or a vicinity of the fundus. A type of the OCT includes a time domain OCT (TD-OCT) and a spectral domain OCT (SD-OCT), and other various types of OCTs have been being developed.
In particular, in recent years, there has been a progress in achieving an even higher resolution owing to a development of a high numerical aperture (NA) of the irradiation laser in the above-mentioned image photographing apparatus for the ophthalmic application.
However, when taking an image of a fundus, the image is taken through optical textures of an eye, such as a cornea and a lens. Therefore, as the resolution is increased, the quality of the taken image has become influenced significantly by aberrations of the cornea and the lens.
In order to cope with this problem, researches on an adaptive optics (AO)-SLO and an AO-OCT have been being progressed, in which the aberration of an eye is measured and an AO function for correcting the aberration is incorporated in an optical system. In general, the AO-SLO and the AO-OCT measure a wave front of the eye by using a Shack-Hartmann wave front sensor system. The Shack-Hartmann wave front sensor system measures the wave front of the eye by irradiating the eye with measuring light and receiving reflected light reflected on the eye with a CCD camera through a microlens array. The AO-SLO and the AO-OCT can take a high resolution image by driving a variable shape mirror and a spatial phase modulator to correct the measured wave front and taking an image of the fundus through the variable shape mirror and the spatial phase modulator. An image photographing apparatus employing two spatial light modulators is disclosed in Japanese Patent Application Laid-Open No. 2011-104332.
In general, when the NA of the irradiation laser is increased to increase the resolution, the amount of the aberration due to the optical textures of the eye, such as the cornea and the lens, is increased and the profile of the aberration becomes complicated. Although the aberration can be corrected by the AO, in order to correct a large aberration or a complicated profile aberration, it is required to measure the aberration with a high resolution and to drive a wave front correction device with a high resolution. In order to measure the aberration with a high resolution and to drive the correction device with a high resolution, a large number of calculations are required, causing a problem of increasing the calculation time.
Although a spatial phase modulator employing liquid crystal can be used to correct the wave front with a high resolution, only a specific polarized component can be corrected due to the nature of the liquid crystal. Therefore, multiple spatial phase modulators are needed to support both polarized components.
In the spatial phase modulator, control values to be instructed may differ even when performing the same wave front correcting due to the modulation characteristic, an initial distortion state, and the like of each individual spatial phase modulator. Therefore, when multiple spatial phase modulators are used, the calculation load is further increased, resulting in a decrease of a feedback speed of the aberration correction.
Further, the wave front measured by the wave front sensor is slightly different depending on the polarization due to a modulation error in the spatial phase modulator and the like. Therefore, the measurement accuracy is degraded when both polarizations are measured at the same time, resulting in a problem that the time to converge the aberration correction feedback is increased or the convergence cannot be obtained up to a sufficiently low aberration.
In particular, improvement of a processing speed is very important because the aberration of the eye requires a high-speed repetition of the aberration correction due to a constant change of a state of tears or a state of visibility adjustment of the eye. The high-speed aberration correction is particularly important when using multiple spatial light modulators as disclosed in Japanese Patent Application Laid-Open No. 2011-104332.